The rostral portion of the nucleus of the solitary tract (rNST) in the brainstem is the first central taste relay. The rNST receives primary afferent projections from facial and glossopharyngeal nerves, which innervate lingual taste buds and papillae and terminate centrally in a topography that reflects peripheral organization. The time course for and molecular factors involved in proliferation and differentiation of rNST cells are not well understood. Given the importance of an organized taste system for proper function, the long-term objective is to determine how the rNST is established in embryonic rat. The proposed research aims are to determine: the development of solitary tract (ST) projections; the developmental time course of proliferation and differentiation of cell components comprising the rNST; the development of biophysical properties and synapses of rNST neurons; specific molecular factors present in relation to rNST development; and, to manipulate the expression of these factors to determine effects on rNST development. Immunofluorescence will be used to identify the time course for development of neurons and glia that compose the rNST in staged embryos. An in vitro brainstem slice preparation will be used to determine development of neuron function. Immunohistochemical and Western blot approaches will be used to identify the expression of Sonic hedgehog (Shh) signaling factors potentially important for proliferation and differentiation in the NST, and cell cycle regulators across developmental stages. Finally, an explant culture system will be used to isolate the brainstem and manipulate in vitro Shh signaling molecules localized to the rNST and surround. The working hypotheses are that during embryonic development, neural precursors migrate from the fourth ventricle and differentiate to form the earliest rNST, where neurons differentiate before glia; that rNST neurons are functional, but with changing properties, in the embryo; and, that Shh signaling regulates the timing of proliferation and differentiation of rNST neural precursors and emerging neurophysiological function in rNST. Results of these experiments will provide an understanding of the early establishment, organization and function of the rNST and will demonstrate possible molecular pathways involved in that development. This will provide an important foundation for understanding the formation of the primary afferent relay of the gustatory system and lay the groundwork for understanding the demonstrated plasticity that occurs in this relay when environmental manipulations take place during embryogenesis. Furthermore, the proposal addresses essential questions in the process of neural patterning in the CNS.